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  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></div></div> <!----></nav>  <ul class="sidebar-links"><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading open"><span>算法</span> <span class="arrow down"></span></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/tech-document/algorithm/" aria-current="page" class="active sidebar-link">两数之和</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><div class="custom-header" data-v-46893521><h1 class="title" data-v-46893521></h1> <div class="name" data-v-46893521>
    作者:
    <span data-v-46893521></span></div> <div class="categories" data-v-46893521>
    分类:
    </div> <div class="tags" data-v-46893521>
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    </div> <div class="date" data-v-46893521>
    创建时间:
    <span data-v-46893521></span></div></div> <h3 id="题目描述"><a href="#题目描述" class="header-anchor">#</a> 题目描述</h3> <p><a href="https://leetcode.cn/problems/two-sum/" target="_blank" rel="noopener noreferrer">leetcode<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <p>给定一个整数数组 nums  和一个整数目标值 target，请你在该数组中找出 和为目标值 target 的那两个整数，并返回它们的数组下标。</p> <p>你可以假设每种输入只会对应一个答案。但是，数组中同一个元素在答案里不能重复出现。</p> <p>你可以按任意顺序返回答案。</p> <h3 id="题目示例"><a href="#题目示例" class="header-anchor">#</a> 题目示例</h3> <div class="language-js extra-class"><pre class="language-js"><code>输入：nums <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">7</span><span class="token punctuation">,</span><span class="token number">11</span><span class="token punctuation">,</span><span class="token number">15</span><span class="token punctuation">]</span><span class="token punctuation">,</span> target <span class="token operator">=</span> <span class="token number">9</span>
输出：<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span><span class="token number">1</span><span class="token punctuation">]</span>
解释：因为 nums<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">+</span> nums<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span> <span class="token operator">==</span> <span class="token number">9</span> ，返回 <span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">]</span> 。
</code></pre></div><div class="language-js extra-class"><pre class="language-js"><code>输入：nums <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">,</span> target <span class="token operator">=</span> <span class="token number">6</span>
输出：<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">]</span>
</code></pre></div><div class="language-js extra-class"><pre class="language-js"><code>输入：nums <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">,</span> target <span class="token operator">=</span> <span class="token number">6</span>
输出：<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span><span class="token number">1</span><span class="token punctuation">]</span>
</code></pre></div><h3 id="暴力解法"><a href="#暴力解法" class="header-anchor">#</a> 暴力解法</h3> <p>两层 for 循环，两两相加，找到之和等于 target 的两个数即可</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> nums <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">7</span><span class="token punctuation">,</span> <span class="token number">11</span><span class="token punctuation">,</span> <span class="token number">15</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
  target <span class="token operator">=</span> <span class="token number">9</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> <span class="token function-variable function">twoSum</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">nums<span class="token punctuation">,</span> target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> len <span class="token operator">=</span> nums<span class="token punctuation">.</span>length<span class="token punctuation">;</span>

  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> len<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> j <span class="token operator">=</span> i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">;</span> j <span class="token operator">&lt;</span> len<span class="token punctuation">;</span> j<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span>nums<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">+</span> nums<span class="token punctuation">[</span>j<span class="token punctuation">]</span> <span class="token operator">===</span> target<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token punctuation">[</span>i<span class="token punctuation">,</span> j<span class="token punctuation">]</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">twoSum</span><span class="token punctuation">(</span>nums<span class="token punctuation">,</span> target<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p><strong>复杂度分析：</strong></p> <ul><li><p>时间复杂度：O(N^2)，其中 N 是数组中的元素数量。最坏情况下数组中任意两个数都要被匹配一次。</p></li> <li><p>空间复杂度：O(1)。</p></li></ul> <h3 id="优化解法-哈希表"><a href="#优化解法-哈希表" class="header-anchor">#</a> 优化解法（哈希表）</h3> <p><strong>思路：</strong></p> <p>每次遍历，判断当前元素和目标值之差（另一个数）是否在哈希表中，</p> <p>如果在直接返回当前元素和另一个数的索引位置</p> <p>如果不在，就将当前元素的值作为 key，索引值作为 value，插入到哈希表中</p> <p><strong>举个例子：</strong></p> <p><code>nums = [2, 7, 11, 15], target = 9;</code></p> <table><thead><tr><th>key(元素值)</th> <th>2</th> <th>7</th> <th>11</th> <th>15</th></tr></thead> <tbody><tr><td>value(元素索引位置)</td> <td>0</td> <td>1</td> <td>2</td> <td>3</td></tr></tbody></table> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> nums <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">7</span><span class="token punctuation">,</span> <span class="token number">11</span><span class="token punctuation">,</span> <span class="token number">15</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
  target <span class="token operator">=</span> <span class="token number">9</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> <span class="token function-variable function">twoSum</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">nums<span class="token punctuation">,</span> target</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> len <span class="token operator">=</span> nums<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
  <span class="token keyword">const</span> map <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> len<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> cur <span class="token operator">=</span> nums<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>
    <span class="token comment">// 可能索引值为0，因此不能直接写map[target - cur]</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>map<span class="token punctuation">[</span>target <span class="token operator">-</span> cur<span class="token punctuation">]</span> <span class="token operator">!==</span> <span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> <span class="token punctuation">[</span>map<span class="token punctuation">[</span>target <span class="token operator">-</span> cur<span class="token punctuation">]</span><span class="token punctuation">,</span> i<span class="token punctuation">]</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    map<span class="token punctuation">[</span>cur<span class="token punctuation">]</span> <span class="token operator">=</span> i<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">twoSum</span><span class="token punctuation">(</span>nums<span class="token punctuation">,</span> target<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
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